Cyclone separators have many uses. A major one is in the cellulose industry for the purification of cellulose fiber suspensions. Generally, a cyclone separator system includes several stages coupled in series, with every stage comprising several cyclone separators connected in parallel, having inlet and outlet chambers in common. Such a cyclone separator system separates the original, highly diluted cellulose suspension into diluted, purified fibers, called the "light fraction", and thickened impurities, called the "heavy fraction". As modern process technology has advanced, cellulose suspension temperatures have increased, causing viscosities to decrease. With decreasing viscosities and the same number of stages, the separating power of a cyclone separator system decreases and more cellulose fibers are discarded with the heavy fraction. A high fiber concentration in the heavy fraction can also cause plugging of the final cyclone separator stage.
Many attempts have been made, some on a commerical scale, to solve problems of fiber loss and plugging. One such attempt entails supplying water under pressure to the individual cyclone separators to dilute the heavy fraction and to wash out the valuable fibers. Water is supplied tangentially near the heavy fraction outlet end of the cyclone separator, or through a channel ending at a radial distance from the wall of the cyclone separator within the heavy fraction outlet end. Another involves discharge chambers, formed like cylinders or cones and provided with a tangential inlet for diluting water, directly conneted to the heavy fraction outlet of the cyclone separator. At best these attempts have solved the plugging problems and reduced the fiber losses, but drawbacks remain. It is very important in a cyclone separator that the pressure conditions prevailing in the inlet and the outlet are correct. Since diluting water has hitherto been fed to every single separator, it has been necessary to adjust the diluting water flow with great accuracy, which is difficult using regular valves. And because a cyclone separator plant consists of several stages each having several cyclone separators arranged in parallel, the diluting water must be distributed absolutely evenly between the different units, a requirement that has proven nearly impossible to satisfy in practice. Furthermore, existing solutions create severe wear problems.
The principle of driving a cyclone separator with diluting water, supplied through a tangential inlet, while a liquid suspension is supplied through another tangential inlet is known as well, e.g. from U.S. Pat. No. 3,503,503. That patent discloses the supply of liquid suspension through a tangential inlet, arranged at right angle to the principal axis of the cyclone separator. It cannot, however, be used successfully to recover fibers from the heavy fraction flow discharging from a cyclone separator system for purification of cellulose suspension. If the objective is to purify the cellulose fibers to a great extent, and at the same time to minimize the waste flow for environmental reasons, the impurities must be collected in a final heavy fraction flow constituting a minor part of the feed flow.